In all the work in both vols. 1 and 2 it has been assumed that the relative magnitudes of the loads acting on a frame have been fixed. It is true that there has been, if only implicitly, some possibility of choice of combination of loads; for example, the wind blowing from left to right might give a more critical loading case than loading in the opposite direction. Similarly, it has been assumed that the designer will have arranged other live loads in the worst possible way, that is, in the way which will lead to the largest values of full plastic moment of the members. For a complex structure more than one pattern of loading may have to be considered; for example, one design might be carried out under dead plus superimposed loading at a certain load factor, say 1·75, and another under dead plus superimposed plus wind loading at a lower factor, say 1.4. The designer would take the more critical of these two cases for any particular structure.

However, once a critical pattern (or patterns) of loading has been fixed, the problem has then been treated as one of statical collapse. For the purposes of analysis, all loads are imagined to be increased slowly and in proportion until collapse of the frame occurs by the formation of a mechanism of collapse at a load factor λc.